Digestive System Functions

Overview of Digestion

• Digestive system: breakdown of ingested food, absorption of nutrients, and passage of waste products
• Two main groups of organs: alimentary canal (nutrition) and accessory digestive organs

Mouth (Oral Cavity) Anatomy

• Vestibule: space between lips externally and teeth and gums internally
• Tongue: attached at hyoid and styloid processes, and by the lingual frenulum
• Frenulum: membrane that secures the tongue to the floor of the mouth, limits movement
• Tonsils: palatine tonsils and lingual tonsil

Pharynx

• Serves as a passageway for air and food
• Food is propelled to the esophagus by two muscle layers (peristalsis)

Esophagus

• Runs from pharynx to stomach through the diaphragm
• Conducts food by peristalsis (slow rhythmic squeezing)
• Passageway for food only (respiratory system branches off after the pharynx)

Stomach

• Acts as a storage tank for food
• Site of food breakdown
• Chemical breakdown of protein begins
• Delivers chyme (processed food) to the small intestine
• Produces 2-3L/day of gastric juice (HCl, enzymes, and mucus)
• Regulated by neural and hormonal factors

Small Intestine

• Site of nutrient absorption into the blood
• Muscular tube extending from the pyloric sphincter to the ileocecal valve
• Approximately 4-8 hours for food to pass through
• Subdivisions: duodenum (5%), jejunum (40%), and ileum (55%)
• Digestion: breaks down double sugars into simple sugars, completes protein digestion, and aids in fat digestion
• Absorption: water is absorbed along the length of the small intestine

Large Intestine

• Absorbs water and eliminates indigestible food from the body as feces
• Does not participate in digestion of food
• Structures: ileocecal valve, cecum, appendix, colon (ascending, transverse, descending, and sigmoidal), rectum, and anus
• Functions: absorption of water, elimination of indigestible food, and production of vitamin K and B
• Propulsion: slow, powerful movements (mass movements) occur three to four times per day

Accessory Digestive Organs

• Salivary glands: produce saliva that aids in chemical digestion

• Pancreas: produces a wide spectrum of digestive enzymes that break down all categories of food

• Liver: produces bile, detoxifies drugs and alcohol, degrades hormones, produces cholesterol, and plays a central role in metabolism### Anatomy of the Liver

• The liver consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament.

• It is connected to the gallbladder via the common hepatic duct.

Carbohydrate Metabolism

• If the body has an abundance of glucose, it is converted into glycogen (glycogenesis).
• If the body lacks sugar, the liver breaks down glycogen into sugar (glycogenolysis).
• Glucose is broken down, releasing chemical energy to form ATP during cellular respiration.

Fat Metabolism

• Most fatty acid metabolism occurs in the liver.
• Fats are broken down into acetic acid, and then oxidized to form CO2, H2O, and ATP.

Protein Metabolism

• Amino acids are used to make ATP only when proteins are excessive or carbohydrates and fats are unavailable.
• Amino acids are oxidized, and ammonia (NH3) is secreted; the rest enter the citric acid cycle.

Nutrition

• A nutrient is a substance used by the body for growth, maintenance, and repair.
• Categories of nutrients:
  • Carbohydrates (simple sugars, starches, fiber)
  • Lipids (triglycerides, phospholipids, fatty acids)
  • Proteins (amino acids)
  • Vitamins
  • Minerals (7 essential minerals: Ca, P, K, S, Na, Cl, Mg)
  • Water

Diseases and Disorders

• Gallstones occur when bile is stored for too long, and fat crystallizes.
• Jaundice occurs when bile enters the bloodstream, and tissues become yellow.

Processes of the Digestive System

• Ingestion: getting food into the mouth
• Food breakdown: mechanical and chemical digestion
• Food movement: peristalsis and segmentation
• Absorption: end products of digestion are absorbed into the blood or lymph
• Defecation: elimination of indigestible substances

Energy and Metabolism

• Energy intake = total energy output (heat + work + storage energy)
• Basal metabolic rate (BMR) is the amount of energy needed to perform essential life activities.
• Factors that influence BMR: surface area, gender, age, and thyroid function.

Urinary System

• Functions:
  • Elimination of waste products
  • Maintenance of blood composition
• Organs: kidneys, ureters, urinary bladder, and urethra

Kidney Structure

• Location: high on the posterior abdominal wall
• Coverings: fibrous capsule, adipose capsule, and renal fascia
• Regions: renal cortex, renal medulla, and renal pelvis

Nephrons

• Functional units of the kidneys
• Consist of renal corpuscle and renal tubule
• Responsible for forming urine
• Over 1 million nephrons in each kidney

Urine Formation

• Processes: filtration, reabsorption, and secretion

• Filtration: nonselective, passive process at the glomerulus

• Regulation of filtration rate: sympathetic nervous system reflexes and hormonal regulation### Renal Function and Urine Production

• Filtration pressure: hydrostatic pressure of blood forces substances through the capillary wall, while net filtration pressure is always positive.

• Hydrostatic pressure of blood is greater than the hydrostatic pressure of the glomerulus capsule and the osmotic pressure of glomerulus plasma.

• The sodium potassium pump reabsorbs 70% of sodium ions in the proximal convoluted tubule (PCT).

The Renin-Angiotensin Mechanism

• Renin is produced by juxtaglomerular cells in response to stimuli such as sympathetic stimulation, decreased blood pressure, and decreased sodium and chloride ions.
• Renin reacts with angiotensinogen to form angiotensin I, which is converted to angiotensin II by the angiotensin I converting enzyme (ACE).
• Angiotensin II acts to vasoconstrict efferent arterioles, increasing blood pressure and maintaining filtration rate.

Reabsorption and Secretion

• Reabsorption: water, glucose, amino acids, and needed ions are transported out of the filtrate into the peritubular capillary cells.
• Most reabsorption occurs in the proximal convoluted tubule (PCT), where microvilli cells act as transporters.
• Active transport of sodium ions occurs along the remainder of the nephron and collecting duct.
• Almost all sodium ions and water are reabsorbed.
• Secretion: hydrogen ions, potassium ions, creatinine, and drugs are removed from the peritubular capillaries and secreted into the filtrate.

Maintaining Water Balance

• Normal amount of water in the human body: 50% in young adult females, 60% in young adult males, 75% in babies, and 45% in older adults.
• Water is necessary for many body functions, and levels must be maintained.
• Distribution of body fluid: intracellular fluid (inside cells), extracellular fluid (outside cells), interstitial fluid, and blood plasma.

The Link Between Water and Salt

• Electrolyte balance affects water movement between compartments.
• Changes in electrolyte balance alter blood volume and blood pressure.
• Aldosterone regulates sodium ion content of extracellular fluid.

Urine Characteristics

• Composition: ~95% water, containing urea, uric acid, electrolytes, and amino acids (trace amounts).
• pH: 4.5 to 8.0, with acidic urine produced during protein-rich diet, starvation, and diabetes, and basic urine produced during bacterial infections and vegetarian diets.
• Sterile, slightly aromatic, and yellow due to the pigment urochrome.

Urinary System

• Ureters: slender tubes attaching the kidney to the bladder, 10-12 inches long and ¼ inch diameter.
• Urinary bladder: smooth, collapsible, muscular sac that temporarily stores urine.
• Urethra: thin-walled tube that carries urine from the bladder to the outside of the body, with a length of 3-4 cm in females and 20 cm in males.

Micturition (Voiding)

• Both internal and external urethral sphincters must open to allow voiding.
• The internal urethral sphincter is relaxed after the bladder stretches to ~200mL, and the external urethral sphincter must be voluntarily relaxed.
• Incontinence: inability to control micturition, and retention: inability to micturate.

Male Reproductive System

• Testes: primary sex organs that produce sperm and secrete androgens such as testosterone.
• Epididymis: comma-shaped, tightly coiled tube that matures and stores sperm cells.
• Ductus deferens: carries sperm from the epididymis to the ejaculatory duct.
• Urethra: carries both urine and sperm.
• Accessory organs: seminal vesicle, prostate gland, and bulbourethral gland.
• External genitalia: penis and scrotum.

This quiz covers the processes of the mouth, including mastication and digestion, and the breakdown of ingested food. Learn about the functions of the digestive system, including mechanical and chemical digestion.